Since the invention of the integrated circuit (IC), the semiconductor industry has experienced rapid growth due to continuous improvements in the integration density of semiconductor devices (e.g., transistors, diodes, resistors, capacitors, etc.) and continuous reduction in the critical dimension (CD) of semiconductor devices. With the continuous reduction of the CD of semiconductor devices, the scale of the gate, source and drain of a transistor decreases accordingly, which results in the decrease of carriers that determine the magnitude of the current in the transistor.
In various techniques to improve the performance of transistors, one method is to apply mechanical stress to the channel of a transistor to increase the carrier mobility and to reduce resistance. Strained silicon transistors are a result of the application of such a method.
On the other hand, the integration improvements and reductions on CD of semiconductor devices are essentially two-dimensional (2D) in nature, in that the volume occupied by the integrated devices is essentially on the surface of the semiconductor wafer. Although dramatic improvements in lithography have resulted in considerable improvement in 2D IC formation, there are physical limits to the density that can be achieved in two dimensions.
In an attempt to further increase circuit density, three-dimensional (3D) ICs have been investigated. In a typical formation process of a 3D IC, two dies are bonded together and electrical connections are formed between each die and contact pads on a substrate. For example, one attempt involved bonding two dies on top of each other. The stacked dies are then bonded to a carrier substrate and wire bonds electrically coupled contact pads on each die to contact pads on the carrier substrate.
In a 3D IC, the two dies may be bonded together on top of each other using various means such as through vias, e.g., through silicon vias (TSVs) or through-substrate vias. Generally, a through via is formed by etching a vertical via through a substrate and filling the via with a conductive material, such as copper.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.